1. Field of the Invention
The present invention relates to oscillators which include oscillator circuits and amplifier circuits for amplifying oscillation signals output from the oscillator circuits.
2. Description of the Related Art
FIG. 3 shows a conventional oscillator. An oscillator circuit 50 includes an oscillator transistor 51 and a resonant circuit 52. The collector of the oscillator transistor 51 is grounded at high frequencies by a DC-blocking capacitor 53. The collector of the oscillator transistor 51 is connected to a power supply terminal 55 through a choke inductor 54. The emitter is grounded by an emitter bias resistor 56 and an inductance device 57 which is connected in series to the emitter bias resistor 56. A feedback capacitor 58 is connected between the base and the emitter, and a feedback capacitor 59 is connected between the emitter and the ground. A bias voltage is applied to the base by base bias resistors 60 and 61.
The resonant circuit 52 includes a microstrip line 52a, a varactor diode 52b, and the like. One end of the microstrip line 52a and the anode of the varactor diode 52b are grounded. The other end of the microstrip line 52a is connected to the base of the oscillator transistor 51 through a coupling capacitor 52c. The other end is also connected to the cathode of the varactor diode 52b through a DC-blocking capacitor 52d. 
The cathode of the varactor diode 52b is connected to a control terminal 63 through a choke inductor 62. The control terminal 63 is grounded at high frequencies by a DC-blocking capacitor 64. Voltage for changing the capacitance of the varactor diode 52b is applied to the control terminal 63.
An amplifier circuit 70 includes an amplifier transistor 71. The collector of the amplifier transistor 71 is grounded by a capacitor 73. The collector is also connected to an output terminal 75 through a coupling capacitor 74.
A coupling capacitor 76 is connected between the emitter of the oscillator transistor 51 and the amplifier transistor 71. An oscillation signal is input to the base of the amplifier transistor 71.
The oscillation frequency is changed by changing the voltage applied to the control terminal 63.
In the above-described conventional oscillator, an oscillation signal is output from the emitter of the oscillator transistor 51. Since the voltage of the emitter can vary as much as approximately 0.3 V in accordance with a change in the ambient temperature, the level of the oscillation signal will change accordingly. Thus, a problem results in that the level of the oscillation signal input to the amplifier circuit 70 varies.
The emitter of the oscillator transistor 51 is the point at which the oscillator transistor 51 is connected to the two feedback capacitors 58 and 59. When the impedance of the amplifier circuit 70, which is a load, changes in accordance with a change in the oscillation frequency, that is, when there is load fluctuation, the substantial feedback capacitance between the base and the emitter of the oscillator transistor 51 and the substantial feedback capacitance between the emitter and the collector change. A change in the amount of feedback results in a decrease in the stability of oscillation and the level of the oscillation output. Since the level of the oscillation output obtained is divided between the two feedback capacitors 58 and 59, the level is low.
Accordingly, it is an object of the present invention to suppress variations in the level of an oscillation signal due to temperature and to obtain a large oscillation output level by performing stable oscillation without changes in the amount of feedback caused by load fluctuation.
According to the present invention, the foregoing objects are achieved through provision of an oscillator including an oscillator circuit which includes an oscillator transistor and a resonant circuit, the oscillator circuit being capable of changing an oscillation frequency; and an amplifier circuit for receiving an oscillation signal output from the oscillator circuit and amplifying the oscillation signal. The resonant circuit is connected at high frequencies between the base and the collector of the oscillator transistor. The oscillation signal is output from the resonant circuit.
The oscillation signal may be output from one end of the resonant circuit.
The collector of the oscillator transistor may be grounded at high frequencies, and the oscillation signal may be output from the base of the oscillator transistor.
The collector of the oscillator transistor may be grounded at high frequencies. The resonant circuit may include a resonant line. One end of the resonant line may be grounded, and the other end of the resonant line may be connected to the base of the oscillator transistor. The oscillation signal may be output from a position at the middle of the resonant line in the longitudinal direction.
The amplifier circuit may include an amplifier transistor. The emitter of the amplifier transistor may be connected to the collector of the oscillator transistor, and a common bias current may be directed to flow through the amplifier transistor and the oscillator transistor. The oscillation signal may be input to the base of the amplifier transistor.
According to the present invention, it is possible to minimize variations in the level of an oscillation signal caused by temperature change. Even when load fluctuation is caused by changing an oscillation frequency, the substantial feedback amount between the base and the emitter of an oscillator transistor does not change, nor does the substantial feedback amount between the emitter and the collector change. Thus, stable oscillation can be achieved. The level of an output oscillation signal is increased. A grounded-collector oscillator transistor can achieve stable oscillation. Since impedances of an amplifier circuit can be matched easily, it is possible to efficiently input oscillation power to an amplifier transistor. With a grounded-collector oscillator circuit and a grounded-emitter amplifier circuit, the current consumption is reduced and the gain is increased.